Page "Glory (Buffy the Vampire Slayer)" Paragraph 11

After recovering from a mental breakdown caused by Glory's kidnapping of Dawn, Buffy decides to attack Glory with everything she has: Willow's magic spell to restore Tara's sanity stolen by Glory, at the same time weakening Glory's mind ; the Dagon Sphere ( a weapon created by the monks that causes physical pain to Glory ); Buffy's robot double ( the " Buffybot ") originally Spike's sex toy ; Olaf, The Troll God's Hammer ( with which Buffy is able beat Glory in battle and severely weaken her albeit temporarily, due to the hammer being a weapon of the gods ); and eventually a wrecking ball, commandeered by Xander.

Meanwhile, Spike, Giles, and Anya attack her minions.

However, the final spell that opens the rift between dimensions had already started by one of Glory's disciples-Doc-by cutting Dawn with a knife and making her bleed ( the act of which opens a portal to the other dimensions ).

Ultimately, after killing Doc, Buffy realises that because the monks created Dawn from her essence, they share the same blood which means not only Dawn's blood but her own can close the portal, but it would also require her to die ( her blood must stop flowing for the gateway to close ).

After saying goodbye to her sister, Buffy jumps into the portal that is opening, instantly dying and closing the gateway.